The 4" basin inlet centerline is 11" vertical below the slab/cover & toilet flange. Do I need to worry about this small vertical drop and supply a aux vent for the 3" toilet line? With toilets I'm not sure if you have to worry about trap siphoning like you do with other fixtures.

Your system may work without a vent for the toilet, but an inspector would not approve it, and if you had some peculiar reason that one would be absolutely necessary, it would be difficult to install afterwards. It should be very simple to install a vent during the construction, and in some cases, depending on the fixture locations, it might not ever require any additional piping.

I'm curious why it wouldn't be inspector-approved. My intepretation was that the toilet was well within the critical distance to the vent in the basin.

I am planning on connecting the 2" shower drain/vent circuit via a wye between the toilet and the basin (in fact this would occupy most of the space between the toilet and basin fittings). Are you thinking of using this as a wet vent when you mention a vent potentially not requiring additonal piping?